1. Field of the Invention
The present invention relates to an ink jet printing apparatus and a method for a print head recovery.
2. Description of the Related Art
An ink jet printing apparatus performs a printing operation by ejecting ink onto a print medium from an array of nozzles in a print head. A variety of methods are available for ejecting ink from the nozzles. Typical methods include a bubble-jet printing method and a piezoelectric method. The bubble-jet printing method energizes heaters provided one in each of the nozzles according to drive pulses to generate and apply a heat energy to ink in each nozzle, forming a bubble in ink in the nozzle through film boiling so that the bubble as it expands in the nozzle expels by its pressure a predetermined amount of ink from the nozzle. The bubble-jet printing method, however, has a drawback that, when ink is ejected, a part, though small, of the bubble that was generated by film boiling may remain in an ink path. This residual bubble is moved along the flow of ink and accumulated in an ink chamber. Each time the ejection operation is performed, a residual bubble accumulates. As the printing operation continues, the volume of accumulated residual bubbles increases, with small bubbles combining together into larger ones.
When the residual bubbles exceed a certain volume, a printing failure occurs. For example, a residual bubble that has grown large in the ink chamber interferes with a smooth flow of ink and may prevent the ink from being supplied well to the nozzle side, resulting in an ejection failure. Conventional measures to cope with such a printing failure include the following recovery operation.
This recovery operation involves counting the number of ejections executed after the start of a printing operation to determine the accumulated number of ejections for each print head and, when the accumulated number of ejections reaches a predetermined value, temporarily stopping the printing operation to perform a variety of recovery operations such as an ink suction operation and a preliminary ejection operation (see Japanese Patent Application Laying-Open No. 8-132648 (1996)).
As described above, the conventional recovery operation determines an execution of the recovery operation based on the accumulated number of ejections for the entire print head. However, depending on a structure of the ink chamber in the print head, some of the nozzles in the print head easily build up bubbles while others do not. In other words, bubbles do not accumulate uniformly over the entire print head. More specifically, those nozzles close to an ink supply port do not easily build up bubbles. Since the residual bubbles produced in the nozzles near the ink supply port are carried away by the ink being supplied from the ink supply port, the residual bubbles tend to accumulate more at locations farther away from the ink supply port. And at ends of the nozzle column, the portions which are farthest from the ink supply port, the residual bubbles are most likely to build up.
In a long print head such as a full-line head, only a part of nozzles of the print head may be used frequently depending on an image pattern being printed. Even if the total number of ejections for the print head as a whole is small, those nozzles that are put to concentrated use develop large bubbles, causing ejection failures.
Despite the fact that the buildup of residual bubbles varies according to the nozzle positions and that not all the nozzles of the print head are operated uniformly, the prior art executes the recovery operation when the total number of ejections for the print head as a whole reaches a predetermined threshold value. As a result, the recovery operation cannot in some cases be performed at an appropriate timing. To alleviate this problem it has been a conventional practice to determine the threshold value by performing printing operations under a variety of conditions that cause printing failures and which change depending on the liquid chamber structure in the print head and an image pattern being printed, and then selecting a threshold value for the worst condition. This requires the recovery operation to be performed frequently and ink is therefore wasted.